The Virgo interferometer is a large interferometer designed to detect gravitational waves predicted by Einstein's general theory of relativity. Virgo is a Michelson interferometer that is isolated from external disturbances: its mirrors and instrumentation are suspended and its laser beam operates in a vacuum. The instrument's two arms are three kilometers long and located in Santo Stefano a Macerata, near the city of Pisa, Italy. The interferometer is named for the Virgo Cluster of about 1,500 galaxies in the Virgo constellation, about 50 million light-years from Earth.

Virgo is part of a scientific collaboration of laboratories from six countries: Italy and France, the Netherlands, Poland, Hungary and Spain. Other interferometers similar to Virgo have the same goal of detecting gravitational waves, including the two LIGO (Laser Interferometric Gravitational-wave Observatory) interferometers in the United States (at the Hanford Site in Washington State and in Livingston, Louisiana).

The gravitational force: Among all the forces of Nature, the gravitational force is the one that has been known to man for the longest time. One of its fundamental properties -that all bodies fall with the same acceleration- was recognized by Galileo at the beginning of the seventeenth century. Towards the end of the same century, Newton established the universal gravitation law connecting the force responsible of the fall of bodies to the gravitational force between planets. Finally, Einstein, with the theory of general relativity, connected the gravitational field with the structure of space-time. Nevertheless, we dispose of very few elements on properties of gravitational force in particular in extreme conditions such as during the primordial explosion or the collision of black holes. Contrarily to what one could believe the gravitational force is, among the fundamental forces, the less known. 1)

Einstein's theory predicts the existence of gravitational waves which are perturbations of the gravitational field spreading out in space at the speed of light, like ripples on the surface of a pond. However, while electromagnetic radiations (for example visible light) can be absorbed completely by matter, gravitational waves can travel through space without being absorbed neither by stars, nor by interstellar matter.

This very low interaction, together with the weakness of the gravitational force makes the detection of gravitational waves extraordinarily difficult. Actually, after 30 years of active research, we only have an indirect proof of their existence. It has not yet been possible to detect gravitational waves directly, this remains one of the major challenges of experimental physics.

Some background: The Virgo project was approved in 1993 by the French CNRS and in 1994 by the Italian INFN, the two institutes at the origin of the experiment. The construction of the detector started in 1996 in the Cascina site near Pisa, Italy. 2)

In December 2000, CNRS and INFN created the European Gravitational Observatory (EGO consortium), later joined by the Netherlands, Poland, Hungary and Spain. EGO is responsible for the Virgo site, in charge of the construction, the maintenance and the operation of the detector, as well as of its upgrades. The goal of EGO is also to promote research and studies about gravitation in Europe. By December 2015, 19 laboratories plus EGO were members of the Virgo collaboration.

In the 2000s, the "initial" Virgo detector was built, commissioned and operated. The instrument reached its design sensitivity to gravitational wave signals. This long-term endeavor allowed the technical choices made to build Virgo to be validated; it also showed that giant interferometers are promising devices to detect gravitational waves in a wide frequency band. 3) However, the initial Virgo detector was not sensitive enough to achieve such a detection. Therefore, it was decommissioned from 2011 in order to be replaced by the "advanced" Virgo detector which aims at increasing its sensitivity by a factor of 10. The advanced Virgo detector benefits from the experience gained on the initial detector and from technological advances since it was made. 4)

The construction of the initial Virgo detector was completed in June 2003 and several data taking periods followed between 2007 and 2011. Some of these runs were done in coincidence with the two LIGO detectors. Then a long upgrade to the second generation detector, called Advanced Virgo, started; its aim is to reach a sensitivity one order of magnitude better than the initial Virgo detector, allowing it to probe a volume of the Universe 1,000 times larger, making detections of gravitational waves more likely.

Advanced Virgo started commissioning in 2016, joining the two advanced LIGO detectors ("aLIGO") for a first "engineering" observing period in May and June 2017. 5) On 14 August 2017, LIGO and Virgo detected a signal, GW170814, which was reported on 27 September 2017. It was the first binary black hole merger detected by both LIGO and Virgo. 6)

Goals: The first goal of Virgo is to directly observe gravitational waves, a straightforward prediction of Albert Einstein's general relativity. 7) The study over three decades of the binary pulsar 1913+16, whose discovery was awarded the 1993 Nobel Prize in Physics, led to indirect evidence of the existence of gravitational waves. The observed evolution over time of this binary pulsar's orbital period is in excellent agreement with the hypothesis that the system is losing energy by emitting gravitational waves. 8) The rotation motion is accelerating (its period, reported in 2004 to be 7.75 hours, is decreasing by 76.5 microseconds per year) and the two compact stars get closer by about three meters each year. They should coalesce in about 300 million years. But only the very last moments preceding that particular cosmic collision will generate gravitational waves strong enough to be visible in a detector like Virgo. This theoretical scenario for the evolution of Binary Pulsar B1913+16 would be confirmed by a direct detection of gravitational waves from a similar system, the main goal of giant interferometric detectors like Virgo and LIGO.

On the longer term, after accomplishing the primary goal of discovering gravitational waves, Virgo aims at being part of the birth of a new branch of astronomy by observing the Universe with a different and complementary perspective than current telescopes and detectors. Information brought by gravitational waves will be added to those provided by the study of the electromagnetic spectrum (microwaves, radio waves, infrared, the visible spectrum, ultraviolet, X-rays and gamma rays), of cosmic rays and of neutrinos. In order to correlate a gravitational wave detection with visible and localized events in the sky, the LIGO and Virgo collaborations have signed bilateral agreements with many teams operating telescopes to quickly inform (on the timescale of a few days or a few hours) these partners that a potential gravitational wave signal has been observed. These alerts must be sent before knowing whether the signal is real or not, because the source (if it is real) may only remain visible during a short amount of time.

The Gravitational Wave Universe:

For centuries, mankind has observed the sky with the naked eye, and in recent times with bigger and bigger telescopes. Today we study the universe mainly by observing visible light and other forms of electromagnetic radiation, such as radio waves, X rays, and gamma rays. However, recently we have learnt to also exploit other cosmic "messengers", such as high-energy particles including cosmic rays and neutrinos. With Advanced Virgo, we are attempting to detect a new type of astrophysical messenger, detected for the first time in September 2015: gravitational waves. 9)

Gravitational waves are ripples in the fabric of spacetime, and are expected to be emitted by spinning stars in binary systems, black holes, massive stellar explosions, and even shortly after the birth of the Universe, the Big Bang. Most likely there are also as yet unknown astrophysical sources, which must enlarge this list: General Relativity states that any accelerated motion of mass that does not have spherical nor cylindrical symmetry could be a source of gravitational waves.

Advanced Virgo can detect gravitational waves with frequencies between 10 Hz and 10000 Hz. These are the same frequencies as the sound waves that are audible by humans. Therefore, any signal measured by Advanced Virgo can be sent to a loudspeaker, and we can hear the symphony of the Universe.

Coalescences of black holes and neutron stars: When two compact and massive objects such as black holes and neutron stars spin one around the other, they slowly get closer and closer until they finally merge, forming a black hole. This phenomenon is known as coalescence. Such binary systems lose energy by the emission of gravitational waves with increasing frequency and increasing amplitude throughout their long lifetimes. The first experimental evidence for the emission of gravitational waves came from the study of the binary neutron star PSR 1913+16. Russel Hulse and Joseph Taylor were awarded the Nobel Prize for Physics in 1993 for observing that this binary star loses energy exactly as expected by the emission of gravitational waves. Several other binary stars discovered successively behave in the same way.

During the final stage of the merging phase, the gravitational waves are strong enough to be detected by Advanced Virgo just a few seconds before their collision, when they spin 10, 100, 1000 times per second around each other! Such collisions are very rare in a single galaxy, but Advanced Virgo will be searching for them in millions of galaxies. This should enable us to find a handful of them every year. Detecting binary systems will help us to understand how compact astrophysics objects interact, and if such violent collisions are related to the gamma-ray bursts detected by satellites since decades. The first ever detected gravitational wave (called GW150914) came from a binary black hole merger, and it already teached us that black holes as massive as 30 and 60 solar masses do exist!

Figure 2: Simulated time series of the amplitude of a gravitational wave generated by the coalescence of two neutron stars located 20 Mpc away from Earth. This gravitational wave would be seen during only 30 s in the Virgo data (image credit: LIGO-Virgo Collaboration)

Virgo interferometer

Virgo is a Michelson laser interferometer with two orthogonal arms each 3 kilometers long. A beam splitter divides the incident laser beam into two equal components sent into the two arms of the interferometer. In each arm, a two mirrors Fabry-Perot resonant cavity extends the optical length from 3 to about 100 kilometers because of multiple reflections and therefore amplifies the tiny distance variation caused by a gravitational wave. The two beams of laser light coming from the two arms, are recombined out of phase on a detector so that, in principle, no light reaches the detector. The variation of the optical path's length, caused by the changing distance between the mirrors, produces a very small phase shift between the beams and, thus, a variation of the luminous intensity, which is proportional to the wave's amplitude. 10)

Figure 3: Optical configuration of the first generation Virgo detector. On the schematics one can read the level of magnitude of the power stored in the various cavities (image credit: EGU)

However, in this scheme, a large fraction of the light is sent back toward the laser. In order to further increase the power, this light is sent back to the interferometer by a recycling mirror, in phase with the incident beam thus increasing the light power that can reach several tens of kilowatts in the Fabry-Perot resonant cavities. A high light power is important because it allows to improve the sensitivity of the interferometer. With these resonant cavities coupled together, the interferometer can be seen as a giant light trap. If the optics would be perfect and the mirrors perfectly stable, no light should normally reach the detector except when the interferometer plane is crossed by a gravitational wave. The quality and stability of the optics represent therefore one of the major challenges of the interferometer.

Virgo is sensitive to gravitational waves in a wide frequency range, from 10 to 10,000 Hz. This should allow the detection of gravitational radiation caused by the coalescence of binary systems (stars or black holes), pulsars and those produced by supernovae in the milky way and in outer galaxies, for instance from the Virgo cluster, hence the name of the project.

Virgo runs day and night, listening to all signals that arrive at any time from any part of the universe. The data cominig from the interferometer as well as the ancillary data necessary to its control (4Mbytes/s) are subjected to a preliminary analysis for quality check and a quick detection of potentially interesting anomalous signals. The data is then put at the disposal of the scientific collaboration, for a deeper analysis.

A full description of the Virgo interferometer detection principles and its builtup is provided in references 4), 5) and NO TAG#. In addition, the Virgo interfrometer is based on the same principles as the LIGO interferometer, described in the LIGO file.

• March 26, 2019: The Virgo and LIGO detectors are ready to start the new Observing run called O3, lasting a whole year. The hunt for gravitational waves is set to start on April 1st when the European Virgo detector, based in Italy at the European Gravitational Observatory (EGO), and the LIGO twin detectors, located in the state of Washington and Louisiana (USA), will start to take data becoming together the most sensitive gravitational wave observatory to date. 11)12)

- During a one-year period the LIGO and Virgo Collaborations will register science data continuously, and the three detectors will operate as a global observatory. Since August 2017, the end of the second observation run O2, the two collaborations have intensively worked on their interferometers to improve the sensitivity and reliability. Scientists have also improved their offline and online data analysis and developed further the procedures for releasing Open Public Alerts: these will within minutes notify the physics and astronomy community when a potential gravitational-wave event is observed.

- "With our three detectors now operational at a significantly improved sensitivity, the global LIGO-Virgo detector network is expected to make several new detections. Moreover it will allow precise triangulation of the sources of gravitational waves. This will be an important step towards our quest of multi-messenger astronomy", says Jo van den Brand of Nikhef (the Dutch National Institute for Subatomic Physics) and VU University Amsterdam, who is the spokesperson for the Virgo collaboration.

- "Going from the pioneering era that led to the historical discovery to the present observatory era, where the interferometer and the infrastructure will have to operate faultlessly for 24 hours, seven days in a week for a whole year was and continues to be a daunting challenge;" says Stavros Katsanevas, Director of EGO, "I have confidence however that we will face this challenge with the same success we faced the previous one."

- The detector sensitivity is commonly given in terms of the distance at which it can observe the merger of a binary neutron star system. "During O2 Advanced Virgo could observe neutron star events up to a distance of 88 million light years", says Alessio Rocchi, researcher at INFN and Virgo's commissioning coordinator. "Both LIGO and Virgo Collaborations have been working to improve the sensitivity of the detectors, also exploiting the upgrades installed on the interferometers. With respect to O2, the Virgo sensitivity has improved by about a factor of 2, which means that the volume of the observable Universe has increased by a factor of 8", concludes Rocchi.

- "The quality of the data collected by the instruments is a determining factor to detect gravitational-wave signals buried into noise and to measure their properties", says Nicolas Arnaud, CNRS researcher currently seconded to EGO and Virgo detector characterization coordinator. "A lot of progress has been made in that direction since O2, thanks to the combined effort of the collaboration as a whole, from the instrumentalists to the data analysts".

- The scientific output of observation run O3 is expected to be tremendous and it will potentially reveal new exciting signals coming from new sources such as the merger of mixed binaries made by a black hole and a neutron star. O3 will also target long lasting gravitational waves produced for instance by spinning neutron stars which are not symmetric with respect to their axis; however, the detection of such signals is still an enormous challenge and the LIGO and Virgo Collaborations are raising up to it.

- Furthermore, thanks to the upgrades of Virgo and LIGO, signals for the merger of binary black holes, such as for GW150914 the first gravitational-wave event ever detected, are expected to become quite common, perhaps up to one per week. Scientists also expect to observe several binary neutron star mergers, such as GW170817 which opened the era of multi-messenger astronomy as well as providing insights into binary evolution, nuclear physics, cosmology and fundamental physics.

- "The new software we have built is able to send Open Public Alerts within five minutes", says Sarah Antier, postdoc at the Université Paris Diderot and responsible of the low latency program for the Virgo Collaboration. "This will allow to follow-up the gravitational wave signal with neutrino and/or electromagnetic searches, that may lead to multimessenger discoveries. Observations of many signals as we expect during O3 will give us a census of the population of stellar mass remnants and a better understanding of the violent universe."

- Since August 2017 both LIGO and Virgo have been updated and tested. In particular Virgo has fully replaced the steel wires which were used in O2 to suspend the four main mirrors of the 3 km long interferometer: the mirrors are now suspended with thin fused-silica (‘glass') fibres, a procedure which has allowed to increase the sensitivity in the lowmedium frequency region, and has a dramatic impact in the capabilities to detect mergers of compact binary systems. A second major upgrade was the installation of a more powerful laser source, which improves the sensitivity at high frequencies. Last but not least squeezed vacuum states are now injected into Advanced Virgo, thanks to a collaboration with the AEI (Albert Einstein Institute) in Hannover, Germany. This technique takes advantage of the quantum nature of light and improves the sensitivity at high frequencies.

- Squeezing is a major upgrade also implemented in the two LIGO interferometers in the US for this next observation run. Moreover, the laser power has been doubled to more precisely measure the effect of passing gravitational waves. Other upgrades were made to LIGO's mirrors at both locations, with a total of five of eight mirrors being swapped out for better-performing versions.

Figure 5: The image shows the rear-side view of a suspended mirror. The coating reflects the Virgo near-infrared laser beam, but is transparent in the visible range. A scientist is finally releasing the safety stops used during installation. The 42kg mirror is suspended from four thin fused-silica fibres, which are bonded to the sides of the mirror (image credit: EGO/Virgo Collaboration/Perciballi)

- "We had to break the fibers holding the mirrors and very carefully take out the optics and replace them," says Calum Torrie, LIGO's mechanical-optical engineering head at Caltech. "It was an enormous engineering undertaking."

- During O3 the LIGO and Virgo Collaborations will continue to communicate new findings to the scientific community as well as to the general public. Furthermore scientists will keep on extracting all possible physics results from the data.

- The global LIGO-Virgo network will provide prompt localizations of gravitational-wave signals and will release confident events publicly through the Open Public Alert system to maximize the science that the entire scientific community can do with the gravitational-wave detections and to minimize the chance of missing any electromagnetic or neutrino counterparts.

- The Japanese detector KAGRA (Kamioka Gravitational Wave Detector) is expected to join the global LIGO-Virgo network in the last part of O3, extending the detection and pointing capabilities of the global network.

• On March 4, 2019, the Advanced LIGO and Advanced Virgo detectors are starting their 14th Engineering Run. ER14 will last at least four weeks: it will be the final test before the start of the 3rd LIGO-Virgo Observation Run (O3). During the first half of the run activities will be scheduled that might still improve the three interferometres in the network. ER14 will allow scientists to perform long-term tests of the detector stability, as well as to check the readiness of the software that analyses data in real time. No automatic open public alerts will be issued during this period: any significant candidate trigger identified in the data would have to pass human vetting first. 13)

- The successful completion of ER14 will lead the path to the one year-long O3 science run: O3 will be the longest period during which the global network of advanced gravitational-wave detectors will be observing the Universe with unsurpassed sensitivity. Since the end of the O2 science run in August 2017, the LIGO and Virgo Collaborations have continued working on their instruments to improve their performance. The figure on the left shows how the sensitivity of Advanced Virgo has progressed steadily over the past months.

• February 28, 2019: The O2 observing run began on the 30th of November, 2016 and ended on the 25th of August, 2017. This was the second observing run of Advanced LIGO, and the first observing run of Advanced Virgo, which joined O2 on the 1st of August, 2017. 14)

- The release includes over 150 days of recorded data from each of the two LIGO observatories, as well as 20 days of recorded data from Virgo, making this the largest data set of 'advanced' gravitational-wave detectors to date. Observations in O2 include seven binary black hole mergers, as well as the first binary neutron star merger observed in gravitational waves, all recently published in the GWTC-1 catalogue. Along with the strain data, the release contains detailed documentation and links to open-source software tools. As with previous data releases, the O2 data set should be useful for both scientific investigations and educational activities.

Figure 7: This graph shows the sensitivity achieved during O2 of the three detectors in the network (image credit: Virgo LIGO Collaboration)

• On 1 December 2018, scientists attending the Gravitational Wave Physics and Astronomy Workshop in College Park, Maryland, presented new results from searches for coalescing cosmic objects, such as pairs of black holes and pairs of neutron stars, by the LIGO and Virgo detectors. The LIGO and Virgo interferometers have now confidently detected gravitational waves from a total of 10 stellar-mass binary black hole mergers and one merger of neutron stars, which are the dense, spherical remains of stellar explosions. Seven of these events had been reported before, while four of the black hole detections are newly announced. 15)16)

- From September 12, 2015, to January 19, 2016, during the first LIGO observing run since undergoing upgrades in a program called Advanced LIGO, gravitational waves from three binary black hole mergers were detected. The second observing run, which lasted from November 30, 2016, to August 25, 2017, yielded a binary neutron star merger and seven additional binary black hole mergers, including the four new gravitational wave events being reported now. The new events are known as GW170729, GW170809, GW170818 and GW170823 based on the dates on which they were detected.

- The Virgo interferometer joined the two LIGO detectors on August 1, 2017, while LIGO was in its second observing run. Although the LIGO-Virgo three-detector network was operational for only three-and-a-half weeks, five events were observed in this period. Two events detected jointly by LIGO and Virgo, GW170814 and GW170817, have already been reported.

- One of the new events, GW170818, detected by the global network formed by the LIGO and Virgo observatories, was precisely pinpointed in the sky. The sky position of the binary black holes, located about 2.5 billion light-years from Earth, was identified with a precision of 39 square degrees. That makes it the next best localized gravitational-wave source after the GW170817 neutron star merger.

Figure 8: This illustration shows the localizations of the various gravitational-wave detections in the sky. The triple detections are labelled as HLV from the initials of the three interferometers (LIGO-Hanford, LIGO-Livingston and Virgo) that observed the signals. The reduced areas of the triple events demonstrate the capabilities of the global gravitational-wave network (image credit: Virgo LIGO Collaboration)

• July 17, 2018: As planned, the Virgo and LIGO detectors stopped taking data for the ‘Observation Run 2' - O2 - on the 25th of August, 2017. For both collaborations, this marked the beginning of a new and busy period, which is scheduled to last at least one year (see timelines on the left). During this time, the sensitivity of all three instruments (that is, their ability to detect signals even fainter than those observed in 2015 and 2017 or coming from similar sources, but located further away in the universe) should be improved significantly. Then, in early 2019, the data-taking period – O3 – will start and last for about one calendar year. 17)

- On the Virgo side, three phases were foreseen. First, commissioning activities were undertaken until mid-autumn, in order to improve our knowledge of the detector configuration that was used to take data, and to fix issues identified during the August 2017 run. Indeed, for all large facilities such as Virgo, the golden rule is to disturb the instrument as little as possible while it is running. Only issues preventing the taking of data are promptly fixed and only straightforward improvements are allowed. Therefore, it was only after the end of the O2 run that people working in the Virgo control room were able to carry out many different tests and modify the hardware and software configurations of the detector.

- Instrument upgrades were implemented from the end of November 2017 until the middle of March 2018. Several pieces of hardware were replaced or modified. Notable upgrades included:

1) The installation of a more powerful, ultra-stable input laser: the larger the power circulating in the detector, the more sensitive it is, in particular in the high-frequency range: above a few hundred hertz. With the new laser, the O3 input power is expected to increase by up to a factor of three in comparison with the O2 data-taking period.

2) The replacement of the steel wires suspending the mirrors forming the 3-km long Fabry-Perot cavities - four mirrors in total - with fused silica - ‘glass' - wires. This lower-dissipation material helps in reducing friction at the anchor levels, hence the suspension thermal noise, which represents the dominant fundamental noise impacting upon the Virgo sensitivity in the medium-frequency region, where it is at its best. These fibers have high breaking strengths, but they are fragile, which makes the whole process - from the production of the fibers in a dedicated lab at EGO, to the suspension of the mirror from its superattenuator structure - challenging. In parallel, the vacuum quality has been improved. About a year and half ago, particle contamination of the vacuum caused some fused-silica fibers to break inside the detector. At that time, the mirrors were again suspended from steel wires and the upgrade to fused-silica fibers was postponed to the O2-O3 long shutdown.

3) The addition of a squeezed vacuum source - a 'squeezer' - provided by the Albert Einstein Institute in Hannover, Germany. This instrument, installed at the output port of the interferometer - where the power exists that results from the interference between the laser beams circulating in the 3-km long arms - helps to 'beat' the quantum noise limit, i.e. to reduce the laser shot-noise, which is dominant at high frequency, below its normal level. This counter-intuitive effect is due to the quantum nature of light: any electromagnetic wave, such as the Virgo laser beam, is defined by two quantities, an amplitude and a phase, both of which are fluctuating. The fluctuations of the phase – also known as 'phase noise' – matter more than the amplitude fluctuations for Virgo. With a squeezer, one can move part of the phase noise to the amplitude noise – the Heisenberg principle states that one cannot decrease both fluctuations: if one goes down, the other should increase – and hence improve the instrument sensitivity. This technique has been successfully implemented in GEO 600 and LIGO: this is the first time it will be tried in Virgo.

- Following the completion of all of these upgrades, Virgo is now back in commissioning mode. The first aim is to learn how to control the detector, which has been significantly modified, and then to improve the sensitivity – by at least a factor of two by the end of the year. This will be challenging but the first results are promising: the best O2 sensitivity – the all-time record sensitivity for Virgo – was surpassed in early June.

• October 17, 2017: For the first time, scientists have directly detected gravitational waves — ripples in space and time — in addition to light from the spectacular collision of two neutron stars. This marks the first time that a cosmic event has been viewed in both gravitational waves and light. 18)

- The discovery was made using the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO); the Europe-based Virgo detector; and some 70 ground- and spaceborne observatories.

Figure 10: The image shows the localization of the gravitational-wave (from the LIGO-Virgo 3-detector global network), gamma-ray (by the Fermi and INTEGRAL satellites) and optical (the Swope discovery image) signals from the transient event detected on the 17th of August, 2017. The colored areas show the sky localization regions estimated by the gamma-ray observatories (in blue) and by the gravitational-wave detectors (in green). The insert shows the location of the apparent known galaxy NGC4993: on the top image, recorded almost 11 hours after the gravitational-wave and gamma-ray signals had been detected, a new source (marked by a reticle) is visible: it was not there on the bottom picture, taken about three weeks before the event (image credit: LIGO-Virgo Collaboration)

Figure 11: Ripples of Gravity, Flashes of Light. On Aug. 17, 2017, the Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo detected, for the first time, gravitational waves from the collision of two neutron stars. The event was not only "heard" in gravitational waves but also seen in light by dozens of telescopes on the ground and in space. Learn more about what this rare astronomy event taught us in a new video from LIGO and Virgo (video credit: LIGO-Virgo, Published: 16 October 2017)

• September 27, 2017: A fourth gravitational-wave signal coming from the merger of two stellar mass black holes located about 1.8 billion light-years away was detected on the 14th of August 2017, at 10:30:43 UTC. GW170814 is the first event observed by the global 3-detector network, including not only the two twin Advanced LIGO detectors, but the Advanced Virgo detector as well (Ref. 6)19)20)21)

Following a multi-year upgrade program and several months of commissioning, the Advanced Virgo detector joined the LIGO "Observation Run 2" data-taking period on the 1st of August. The three instruments worked together until the 25th of August.

Figure 12: GW170814 demonstrates the potential of a 3-detector network, both in terms of localization of a source in the sky and in terms of the testing of Einstein's theory of general relativity. The best GW170814 skymaps, computed by an analysis that uses all of the available information from the three instruments, cover just 60 square degrees (to be compared with several hundreds of square degrees for the LIGO-only network) and GW170814 data have allowed the LIGO-Virgo collaboration to probe, for the first time, the polarization of gravitational waves (image credit: LIGO-Virgo Collaboration)

Therefore, GW170814 holds great promise for the future of multi-messenger astronomy. Additional results, based on data from the three-detector network, will be announced in the near future by the LIGO-Virgo Collaboration; the analysis of the data is currently being finalized.

The detected waves—observed on August 14th, 2017 at 10:30:43 UTC (6:30AM EDT) —were produced by a pair of black holes with 31 and 25 solar masses. They merged to produce a spinning black hole of 53 solar masses. Combining the signal from Virgo with the signal observed in the two LIGO observatories improved the sky localization of the source by over a factor of 10.

The Virgo and LIGO Scientific Collaborations have been observing since November 30, 2016 in the second Advanced Detector Observing Run ‘O2' , searching for gravitational-wave signals, first with the two LIGO detectors, then with both LIGO and Virgo instruments operating together since 1 August 2017. Some promising gravitational-wave candidates have been identified in data from both LIGO and Virgo during our preliminary analysis, and we have shared what we currently know with astronomical observing partners. We are working hard to assure that the candidates are valid gravitational-wave events, and it will require time to establish the level of confidence needed to bring any results to the scientific community and the greater public. We will let you know as soon we have information ready to share. 22)23)

Figure 13: The GWevent GW170814 observed by LIGO Hanford, LIGO Livingston, and Virgo. Times are shown from August 14, 2017, 10:30:43 UTC. Top row: SNR time series produced in low latency and used by the low-latency localization pipeline on August 14, 2017. The time series were produced by time shifting the best-match template from the online analysis and computing the integrated SNR at each point in time. The single-detector SNRs in Hanford, Livingston, and Virgo are 7.3, 13.7, and 4.4, respectively. Second row: Time-frequency representation of the strain data around the time of GW170814. Bottom row: Time-domain detector data (in color), and 90% confidence intervals for waveforms reconstructed from a morphology-independent wavelet analysis (light gray) and BBH (Binary Black Hole) models (image credit: LIGO and Virgo Collaboration)

The era of gravitational-wave (GW) astronomy began with the detection of binary black hole (BBH) mergers, by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors, during the first of the Advanced Detector Observation Runs. 24) Three detections, GW150914, GW151226, and GW170104, and a lower significance candidate, LVT151012, have been announced so far. The Advanced Virgo detector joined the second observation run on August 1, 2017.

• August 25, 2017: The Virgo and LIGO Scientific Collaborations have been observing since November 30, 2016 in the second Advanced Detector Observing Run ‘O2', searching for gravitational-wave signals, first with the two LIGO detectors, then with both LIGO and Virgo instruments operating together since August 1, 2017. Some promising gravitational-wave candidates have been identified in data from both LIGO and Virgo during our preliminary analysis, and we have shared what we currently know with astronomical observing partners. We are working hard to assure that the candidates are valid gravitational-wave events, and it will require time to establish the level of confidence needed to bring any results to the scientific community and the greater public. We will let you know as soon we have information ready to share.

Figure 14: The picture shows the Virgo duty cycle during the whole data taking period: we have been taking science data more than 80% of the time over four weeks! (image credit: LIGO-Virgo Collaboration)

14) "LIGO and Virgo have made publicly available the strain data from the O2 observing run. These data are now available through the Gravitational Wave Open Science Center," Virgo News, 28 February 2019, URL: http://www.virgo-gw.eu/

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).